travis: fetch github releases of luarocks.

luarocks.org has intermittent downtime, which makes slingshot
using github projects fail CI checks. Use the github releases
of luarocks for Travis instead, which has much better uptime
ratios.
* .travis.yml (env): Add LUAROCKS_RELEASE.
(install): Fetch luarocks tarball from github.
* .travis.yml: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>